1. Field of the Invention
The present invention relates to a method for adjusting a spectral line width such as E95 bandwidth of a narrow-band laser when using a narrow-band laser as a light source to expose a semiconductor. The E95 bandwidth means a spectral line width of the spectral area of laser light where 95% energy is concentrated. The present invention particularly relates to a method for reducing the deviation in spectral line width such as E95 bandwidth among a plurality of narrow-band laser devices.
2. Description of the Related Art
The recent trend of refining the configuration and increasing the degree of integration of semiconductor integrated circuits has increased the demand for improvement in resolution of a semiconductor exposure tool (hereafter, referred to as the “exposure tool”). For this purpose, the related art tries to decrease the wavelength of light emitted by an exposure light source. Recently, a gas laser device has replaced a traditional mercury lamp as an exposure light source. Such a gas laser device for exposure is for example a KrF excimer laser emitting vacuum ultraviolet light with a wavelength of 248 nm or an ArF excimer laser emitting vacuum ultraviolet light with a wavelength of 193 nm.
Studies are being conducted for next-generation exposure technologies, represented by an immersion exposure technique in which space between a wafer and an exposure lens of an exposure tool is filled with liquid to change the index of refraction to thereby decrease the apparent wavelength of the exposure light source. When the immersion exposure is performed by using an ArF excimer laser as an exposure light source, the wafer is irradiated with vacuum ultraviolet light with a wavelength of 134 nm in the liquid. This technique is referred to as the ArF immersion exposure (or ArF immersion lithography).
A next-next generation exposure light source which is viewed with high degree of expectation is an F2 laser emitting vacuum ultraviolet light with a wavelength of 157 nm. Further, the F2 laser is possibly used as an exposure light source to perform the immersion technique as described above. It is believed that, in this case, a wafer is irradiated with vacuum ultraviolet light with a wavelength of 115 nm.
KrF and ArF excimer lasers have a free running line width as wide as about 350 to 400 pm. The use of these projection lens will cause occurrence of chromatic aberration, resulting in lose of resolution. Therefore, it is necessary to narrow the spectral line width of laser light emitted by the gas laser device until the chromatic aberration is reduced to a negligible level. For this reason, a line narrowing module having a line narrowing element (e.g. etalon or grating) is provided in a laser resonator of the gas laser device, so that the spectral line width is narrowed. A laser whose spectral line width is narrowed is referred to as the “narrow-band laser”. In general, a laser spectral line width is represented by a full width at half maximum. As shown in FIG. 22(a), the term “full width at half maximum (FWHM)” refers to a spectral line width of a part of the laser light spectral where the light intensity is a half of the peak value.
The image formation performance of an exposure tool can be accurately evaluated by an optical simulation calculation method using optical system data of the exposure tool and laser spectral profile. It is known from the results of the optical simulation calculation that the image formation performance of an exposure tool is greatly affected not only by the full width at half maximum of laser light spectral but also by components in the spectral skirts. Therefore, a new definition called E95 bandwidth (also referred to as spectral purity width) has been introduced to define a spectral line width. As shown in FIG. 22(b), the E95 bandwidth is an index indicating a spectral line width of a part of spectral area of laser light where 95% of energy is concentrated. There is a correlation between the E95 bandwidth and image formation performance of an optical system of the exposure tool. The E95 bandwidth is thus required to be suppressed to 0.5 pm or less in order to guarantee a high quality for integrated circuits produced.
The E95 bandwidth and the spectral line width at full width at half maximum can be varied for example by changing the wavefront of laser light. One of techniques to change the laser light wavefront is disclosed in the Patent Document 1 (Japanese Patent Application Laid-Open No. 2000-312048) which relates to a device for changing the curvature of a grating.
However, it has recently been made known that if the value of the E95 bandwidth or the spectral line width at full width at half maximum is either too large or too small in comparison with a designed value for the optical system of the exposure tool, the quality of the integrated circuit pattern is deteriorated. This is described in the Patent Document 2 (U.S. Pat. No. 6,721,340) and the Patent Document 3 (Japanese Patent Application Laid-Open No. 2001-267673).
When a plurality of laser devices are compared, those laser devices do not necessarily have an equivalent spectral line width such as E95 bandwidth even if they have the same configuration. It is rather common that the spectral line width such as E95 bandwidth differs among the plurality of laser devices. FIG. 23 is a histogram showing the E95 bandwidths in a plurality of conventional laser devices. As shown in FIG. 23, the maximum value of E95 bandwidth was 0.450 pm, the minimum value 0.210 pm, the mean value 0.340 pm, and the standard deviation was 0.061 pm. Five out of twenty devices exhibited a variation in the E95 bandwidth exceeding an allowable range of the E95 bandwidth for an optical system of an exposure tool, for example a range of from 0.350 to 0.450 pm. The result revealed that if these five laser devices having an E95 bandwidth exceeding the allowable range were used as an exposure light source, the quality of integrated circuit patterns was deteriorated to such an extent that it is impossible to produce a semiconductor device.
It is believed that the spectral line width such as E95 bandwidth differs among laser devices due to machine differences thereof. The machine differences among laser devices include the followings.
(1) Individual differences among optical elements (line narrowing elements) such as:                i) variation in diffractive wavefront of gratings;        ii) variation in transmission wavefront of prisms; and        iii) variation in position and optical axis among optical elements in a line narrowing module;        
(2) Machine differences in adjustment of laser optical axis such as:                i) variation in chamber discharge position and optical axis when chambers are replaced;        ii) variation in position and optical axis among line narrowing modules;        iii) variation in optical axis among laser resonators;        
(3) Machine differences of laser chambers such as:                i) variation in discharge position        ii) variation in discharge position and discharge state.        
In a practical exposure process of semiconductor device manufacture, laser devices or modules are replaced due to failure or end of service life of the devices. Due to the machine differences as described above, a replacing laser device will have a different spectral line width such as E95 bandwidth from that of a replaced laser device even if they are of a same type. Moreover, the spectral line width such as E95 bandwidth will vary even in a same laser device between before and after maintenance thereof. This means that the spectral line width such as E95 bandwidth is changed as a result of replacement or maintenance of the laser device, and if such change exceeds an allowable range of the spectral line width such as E95 bandwidth for an optical system of the exposure tool, the quality of integrated circuit patterns is deteriorated to such an extent that it is impossible to manufacture a semiconductor device.
The present invention has been made in view of the circumstances described above. It is an object of the prevent invention to suppress variation in spectral line width such as E95 bandwidth due to machine differences caused during manufacture of laser devices and variation in spectral line width such as E95 bandwidth caused by replacement of maintenance of a laser device, and thus to stabilize the quality of integrated circuit patterns formed by a semiconductor exposure tool.